Johnson's making good on a idea he floated in September to #WGDB that he might try to sue to stop an Office of Personnel Management decision allowing for continued employer contributions to members and staff as they move on to the District of Columbia's health care exchange for 2014, as required by the health care overhaul law. Johnson and like-minded supporters say this runs afoul of the intent of the Obamacare law.

"Senator Johnson's lawsuit is an unfortunate political stunt. I am committed to repealing Obamacare, but the employer contribution he’s attacking is nothing more than a standard benefit that most private and all federal employees receive — including the President," Sensenbrenner said. "Success in the suit will mean that Congress will lose some of its best staff and will be staffed primarily by recent college graduates who are still on their parents' insurance."

Sensenbrenner is a longtime House member and former chairman of the Judiciary Committee. He's expressing concerns that were shared publicly and privately by many lawmakers and senior aides about the possible "brain drain" from taking away the employer contribution. Several Republicans, led by Sen. David Vitter of Louisiana, have floated legislative proposals that would accomplish the same goal as the Johnson lawsuit.

Johnson is set to formally announce the suit at a news conference Monday on Capitol Hill. His office said Sunday that the formal court filing would come today. Johnson outlined his plans in an opinion piece published by The Wall Street Journal.

"Senator Johnson should spend his time legislating rather than litigating as our country is facing big problems that must be addressed by Congress — not the courts. All Republicans want to repeal Obamacare, but this politically motivated lawsuit only takes public attention away from how bad all of Obamacare really is and focuses it on a trivial issue. Fortunately, Senator Johnson's suit is likely frivolous and will not achieve the result he’s seeking," Sensenbrenner added.

"I have always respected Congressman Sensenbrenner, but I am disappointed and puzzled by his disagreement with me on an issue that all but two congressional Republicans (including Congressman Sensenbrenner) have voted in favor of — ending the special treatment for members of Congress and their staffs under Obamacare," Johnson said in his own statement.

"By no means do I believe this issue is trivial, or my lawsuit to overturn this injustice is frivolous," Johnson said in his response. "This is an issue of basic fairness that I believe is worth fighting for."